There is IP multicast as packet distribution method used in one-to-many communication or many-to-many communication (hereinafter, referred to as “multicast communication”) performed on the Internet. IP multicast refers to a method of performing packet replication, forwarding processing thereof, member management processing, and multicast distribution tree construction processing in relay apparatus (hereinafter, referred to as “routers”) spread over the Internet.
In IP multicast, in distribution of packets from one or a plurality of senders to one or a plurality of receivers (hereinafter, a set of senders and receivers in multicast communication will be referred to as an “end node”), a router performs packet replication and packet forwarding processing as appropriate. It is thereby possible to save network resources consumed in packet distribution and enable efficient packet distribution.
However, it is indispensable that the router supports IP multicast. Therefore, there is a low possibility that all routers on the Internet support IP multicast and the environment is soon realized that IP multicast can be used on the entire Internet. For the present, it is expected that the Internet includes both networks that support IP multicast and do not support IP multicast.
In the case of performing multicast communication using IP multicast, all end nodes must connect to a network that supports IP multicast. Further, in IP multicast, routers on a path need to accurately process Join/Leave packets in IGMP (Internet Group Management Protocol) of a receiver, and exchange control messages (by routing protocol for IP multicast) between the routers to forward an IP multicast packet accurately to a receiver, and therefore communication is impossible via a router and network that do not support IP multicast, thereby restricting the use of multicast communication of users.
There is application layer multicast, as a packet distribution method to solve the above problem of IP multicast in multicast communication. IP multicast refers to a method implemented primarily on the router, while application layer multicast is a general term for methods implemented on end nodes.
Application layer multicast refers to a method that when one or a plurality of senders performs packet distribution to one or a plurality of receivers, an end node calculates and constructs a multicast distribution tree with other end nodes as branches, and a packet is transmitted, replicated, and forwarded using IP unicast according to the distribution tree. In application layer multicast, since routers are only required to forward IP unicast packets, the current Internet infrastructure can be used without change, and users are not limited in use of multicast communication.
However, since all functions required for multicast communication are concentrated on end nodes including packet replication and forwarding processing thereof, member management processing, and multicast distribution tree construction processing, there is a problem that the processing load on end nodes increases. Further, all packets distribution is performed by IP unicast, and another problem arises of increasing the network resources consumed in packet distribution. From these reasons, application layer multicast is not suitable particularly for multicast communication having a plurality of senders and receivers.
Thus, the problem remains that the processing load on the end node increases in application layer multicast. On the other hand, there is a system where application layer multicast dedicated hardware on a router performs packet replication and packet forwarding processing as a substitute for the end node to reduce the processing load on the end node (for example, refer to Patent Document 1).
However, the above system needs to prepare special routers provided with hardware that supports the system, and is low in possibility of being spread over the Internet. Accordingly, the convenience is not high for users that perform multicast communication.
Patent Document 1: Unexamined Japanese Patent Publication No. 2003-188918